Processes for the treatment of a sour hydrocarbon fraction where the fraction is treated by contacting it with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent at reaction conditions have become well known and widely practiced in the petroleum refining industry. These processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour hydrocarbon fraction to innocuous disulfides--a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fraction as well as naphtha, kerosene, jet fuel, fuel oil, and the like.
A commonly used continuous process for treating sour hydrocarbon fractions entails contacting the fraction with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. The sour fraction and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent--usually air. Sour hydrocarbon fractions containing more difficult to oxidize mercaptans are more effectively treated in contact with a metal chelate dispersed on a high surface area adsorptive support--usually a metal phthalocyanine on an activated charcoal. The fraction is treated by contacting it with the supported metal chelate catalyst at oxidation conditions in the presence of an alkaline agent. One such process is described in U.S. Pat. No. 2,988,500. The oxidizing agent is most often air admixed with the fraction to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in the caustic-wetted state.
The prior art shows that the usual practice of catalytically treating a sour hydrocarbon fraction containing mercaptans involves the introduction of alkaline agents, usually sodium hydroxide, into the sour hydrocarbon fraction prior to or during the treating operation. See U.S. Pat. Nos. 3,108,081 and 4,156,641. The prior art also discloses that quaternary ammonium compounds can improve the activity of these catalytic systems. For example, see U.S. Pat. Nos. 4,290,913 and 4,337,147. In these patents the catalytic composite comprises a metal chelate, an alkali metal hydroxide and a quaternary ammonium hydroxide dispersed on an adsorptive support.
Although the above process has shown commercial success, there are problems associated with the use of alkaline agents. One problem is that phenols and cresols present in the hydrocarbon stream are extracted into the aqueous alkaline solution. Since phenol is on the EPA list of hazardous compounds, the solution containing the phenols is considered a hazardous waste and must be disposed of according to EPA procedures. Also because of the presence of alkali metals, the aqueous waste stream cannot be used in other parts of the refinery owing to possible contamination of vessels or catalysts with the alkali metals.
Applicants have solved the above problems by making the discovery that ammonium hydroxide can be effectively substituted for an alkali metal hydroxide and that a quaternary ammonium salt (where the anion is halide, nitrate, nitrite, sulfate, phosphate, acetate, citrate and tartrate) can be substituted for a quaternary ammonium hydroxide. Thus, applicants' process does not use any strong base. By using ammonium hydroxide no alkali metals are present in the aqueous waste stream, thereby allowing the waste stream to be re-used in other parts of the refinery. If direct disposal is necessary, this stream can be handled more easily than a corresponding alkali metal hydroxide solution because of the low level of dissolved phenols and the ease by which some or all of the ammonium hydroxide may be stripped from the waste stream.
The only prior art reference known to applicants which mentions the use of ammonia is U.S. Pat. No. 4,502,949. This patent discloses a process for sweetening a sour hydrocarbon fraction using a metal chelate catalyst and anhydrous ammonia in the absence of an aqueous phase. There are several differences between the present invention and the '949 reference. First,the '949 specifically states that the ammonia is present in an anhydrous form and is used in the absence of an aqueous phase. In contrast to this, applicants use ammonium hydroxide in an aqueous form. There is no indication in the '949 reference that aqueous ammonium hydroxide would be a good promoter for mercaptan sweetening.
Second, the stability of the catalyst when ammonia is used is only about 60 hours. Although the '949 reference states that this stability is improved versus a process without ammonia, the stability is very poor when compared to a conventional process using an alkali metal hydroxide. In contrast, applicants' data show that the stability of the catalyst in the instant process is several hundred hours (see details infra), i.e., comparable to a conventional process.
The stability and efficiency of a process using ammonium hydroxide and a quaternary ammonium salt other than hydroxide salt is also unexpected based on the knowledge that alkali metal hydroxides successfully promote mercaptan sweetening. The reason for this is that ammonium hydroxide and alkali metal hydroxides have vastly different base properties. Whereas ammonium hydroxide is a weak base with a K.sub.b (dissociation constant) of 1.79.times.10.sup.-5, alkali metal hydroxides are strong bases which are 100% dissociated, K.sub.b .apprxeq.1. Since the first step in the oxidation of mercaptans is to form a mercaptide ion by abstracting a proton using a strong base, it would not be expected that a weak base such as ammonium hydroxide would adequately promote mercaptan sweetening.
The inadequacy of using ammonium hydroxide is shown by U.S. Pat. No. 4,207,173. The object of the '173 patent is the use of a tetra-alkyl guanidine as a promoter for mercaptan oxidation (no alkaline base present). However, in Table 1, column 8, there is presented data comparing sodium and ammonium hydroxide. The data clearly show that using ammonium hydroxide would not provide an acceptable, i.e., sweet, product. Thus, based on the prior art there is no incentive to substitute ammonium hydroxide for sodium hydroxide.
Of additional importance is the fact that the instant process uses a quaternary ammonium salt other than the hydroxide salt. These salts are not strong bases. The instant process, therefore, sweetens sour hydrocarbon fractions without the use of any strong base. Although the '173 reference uses only a tetraalkyl guanidine as a promoter, there is no indication that such a system would have long term stability. In contrast, applicants have shown (see data infra) that use of ammonium hydroxide and a quaternary ammonium salt affords a process which exhibits very little catalyst deterioration.